Three people were killed when a/the (depending on the context) boat capsized/was capsized due to an overload.
The sentence above would be correct.
We need to use the passive voice: the people were killed BUT they died (they died is an active voice sentence).
And a boat capsizes if it is overturned in the water. The active voice is correct. But the passive voice (is capsized) can also be used: if you capsize a boat, or if it capsizes, it turns upside down or onto its side in the water (Macmillan Dictionary).